1. Field of the Invention
In one aspect, the present invention relates to nutrient injection methods for subterranean microbial processes. In another aspect, the present invention relates to nutrient injection methods for microbial enhanced oil recovery (MEOR). In another aspect, the present invention relates to nutrient injection methods for microbially alterating the permeability of subterranean formations.
2. Description of the Prior Art
Various MEOR techniques have been used to provide enhanced oil recovery from oil-bearing subterranean formations. For example, microorganisms have been used in oil-bearing subterranean formations to: produce CO.sub.2 which dissolves in and reduces the viscosity of the oil in the formation; produce organic acids within the formation which dissolve cementing materials and thereby create flow passages within the formation; produce surfactants which operate to reduce interfacial tension between the oil and the rock of the formation; produce solvents, such as ethanol and acetone, within the formation; produce water-soluble polymers within the formation; and/or physically displace oil adhering to sand particles within the formation. Microorganisms have also been used to plug high permeability zones within subterranean formations. This plugging results from in-situ biomass production (i.e., microbial growth and/or expolymer production). Once the high permeability formation zones are plugged, subsequently injected water and/or polymer floods are caused to flow through oil-rich lower permeability zones.
Numerous microorganisms suitable for achieving various microbial objectives in subterranean formations are known in the art. In order to achieve a specific microbial objective, suitable microorganisms can be selected and injected into the subterranean formation. Oftentimes, however, endogenous microorganisms well suited for achieving a particular microbial objective are already present within the formation.
Although nutrient injection is a critical aspect in the activation and control of many subterranean microbial processes, various problems have plagued nutrient injection methods used heretofore. One problem has involved the excessive utilization and depletion of nutrients by microorganisms located at or near the injection borehole. Excessive utilization and depletion of nutrients by microorganisms located in the vicinity of the borehole operates to prevent indepth nutrient distribution and creates excessive plugging in the vicinity of the borehole (i.e., face plugging). Additionally, differential chromatographic retention of individual nutrient components within subterranean formations operates to prevent indepth distribution of complete growth mediums. Differential chromatographic retention of the individual nutrient components results in the development of an incomplete growth medium at some point in the formation. In many formations, differential chromatographic retention prevents the distribution of a complete nutrient medium beyond the proximity of the borehole.
Many of the nutrient injection methods used heretofore have also been susceptible to microbial contamination problems. When a complete nutrient medium capable of supporting microbial activity is present above ground, microbial cells present in the above ground environment can collect and thrive in the nutrient medium. Thus, measures must be taken above ground to maintain the sterility of the nutrient medium. Adequate sterility measures can be difficult and costly, particularly when a large amount of nutrient medium is involved.
Thus, a need exists for a nutrient injection method which addresses these problems.